North Korean soldiers parade in front a portrait of former North Korean President Kim Il-sung during a military parade in Pyongyang's central square in this photo taken by Kyodo on September 9, 2011 marking the 63rd anniversary of the state's founding. Photo: Reuters

The report, based on Japanese government sources, is the most strident of recent claims that China has been involved in helping to arm its wayward ally after earlier allegations Beijing supplied technology.

Four giant trucks capable of transporting and launching ballistic missiles were exported by a Chinese firm last August, the leading Asahi Shimbun said.

The vehicles were likely those on display at the huge military display in April marking the centenary of the birth of the state's founder Kim Il Sung, the Asahi said.

The sale of weapons systems to Pyongyang is banned under UN Security Council resolutions aimed at containing the hermit state's nuclear ambitions.

But at Washington's urging, Tokyo and Seoul have avoided confronting Beijing in a bid to keep North Korea's patron onside in the international effort to tamp down tensions on the peninsula, the paper reported.

Japanese government papers reveal four 16-wheel vehicles were transported aboard a Cambodian-registered ship, which was tracked by spy satellites leaving Shanghai on August 1 and arriving at Nampho in western North Korea three days later.

The vessel then moved on to Osaka where the Japanese coastguard conducted an on-board inspection and discovered documents detailing the export of the vehicles, issued by an agent in Shanghai, the Asahi said.

A report was passed on to the Japanese government's intelligence office, the Asahi said.

Two months ago the United States said it believed China's assurances that it was abiding by sanctions on North Korea after charges that Beijing supplied missile launcher technology.

"China has provided repeated assurances that it's complying fully with both Resolution 1718 as well as 1874. We're not presently aware of any UN probe into this matter," State Department spokesman Mark Toner told reporters.

"I think we take them at their word," Toner said, adding that he was not aware of specific conversations between the United States and China about the launcher.